1. Field of the Invention
The invention relates to a modified extrusion-based 3D print process, in particular a modified fused deposition modeling process for producing multicoloured three-dimensional objects. In particular, the invention relates to a 3D printing process which is superior to conventionally known methods in its ability to produce mechanically stable, multicoloured 3D objects. The process according to the invention is based on coating, upstream of the printing head, of the polymer strand used for producing the actual object, and on fixing of the coating upstream of entry of the polymer strand into the printing head. Downstream of the extrusion process in the printing head, the coating remains predominantly at the surface of the extruded strand. The coating may contain any of colorants and functional additives.
2. Description of the Background
Rapid prototyping or rapid manufacturing processes are manufacturing processes which aim to convert available three-dimensional CAD data directly and rapidly into workpieces, as far as possible without manual intervention or use of moulds.
The scope of rapid prototyping processes has grown to include a variety of processes. These processes can be divided into two groups: laser-based processes and processes without use of a laser.
The best-known laser-based 3D printing process, which is at the same time the oldest, is stereolithography (SLA). In this process, a liquid composition of a radiation-curable polymer is hardened layer-by-layer by using a laser. The person skilled in the art can clearly see that the only possibility with a workpiece produced by this method is subsequent coloring on the surface. This is complicated and time-consuming.
A similar process is Selective Laser Sintering (SLS), in which, by analogy with the SLA, a pulverulent raw material, e.g. a thermoplastic or a sinterable metal, is sintered selectively layer-by-layer by a laser. Again, the first step of this process can only produce single-color, or non-specifically colored, 3D objects. The same applies to the third laser-based process, “laminated object manufacturing”, in which layers of a paper web or plastics foil provided with adhesive are adhesive-bonded to one another and cut by a laser. The subsequent coloring of an object is described in U.S. Pat. No. 6,713,125.
A conventionally known 3D printing process which can also be used for the production of multicoloured objects is the UV ink-jet process. In this three-stage process, a pulverulent material is applied in thin layers, a UV-curable liquid is printed in the form of the respective layer of the subsequent three-dimensional product onto the said layers, and finally the printed layer is hardened by using a UV source. These steps are repeated layer-by-layer.
In EP 1 475 220, variously colored liquids are provided with hardener, and in WO 2008/077850 are also mixed in a chamber directly upstream of the printing process. Selective coloring is thus possible. However, no sharp color transitions are possible, because of the mixing chamber. This type of process moreover lacks sharpness at the limits of the hardening process, and this can reduce surface smoothness, and can sometimes lead to non-uniform coloring. In WO 01/26023, two printing heads are described with variously colored hardener compositions, giving different elasticity properties in the products. However, the number of colors described is not more than two.
WO 2008/075450 describes a variant in which radiant heat, instead of UV light, is used for hardening and variously colored hardener compositions are similarly used.
GB 2419679 discloses a process in which variously colored polymer particles are applied selectively and can be hardened at various wavelengths. This process is extremely complicated, and leads to color definition that lacks sharpness.
In a process in accordance with WO 2009/139395, similar to 3D ink-jet printing, a colored liquid is applied layer-by-layer and printed selectively with a second liquid which leads to a curing reaction with the first liquid. This type of process can only produce a structure of layer-by-layer colors, except in so far as mixing can occur between the uncured layers of liquid.
Another process is Three-Dimensional Printing (TDP). In this process, by analogy with the ink-jet process, pulverulent materials, which preferably are ceramics, are saturated selectively layer-by-layer with the melt of a thermoplastic polymer. After each print layer, a fresh layer of the pulverulent material must be applied. Solidification of the thermoplastic forms the three-dimensional object.
In the process described in US 2004/0251574, the print of the thermoplastic is followed by selective printing with an ink. This process has the advantage of permitting highly selective printing. However, this process has the disadvantage that it is impossible to achieve uniform color definition or bright coloring, since there is no possibility of achieving uniform penetration of the ink into the composite made of the (ceramic) powder and of the binder.
In the process described in EP 1 491 322, two different materials are printed. The first comprises the binder and a colorant which is precipitated on contact with the second material and thus colors the surface selectively. It is thus possible to produce better color properties on the surface of the object. However, there are problems with the uniform mixing of the two materials and with the complicated two-stage process. There is no description of how, or whether, it is possible to ensure that good color definition is achieved with a multicolor print.
In U.S. Pat. No. 6,401,002, various liquids are used with different inks and the binder. The said liquids are either applied separately dropwise or combined by way of connecting lines in a nozzle upstream of the dropwise application process. The person skilled in the art is aware that neither procedure gives ideal color definition. In the former, the mixing of the inks takes place in viscous liquids on the surface. This mixing is therefore rarely complete. In the second procedure, pressure differences in the connecting lines can lead to extreme color variation.
Among printing processes for the production of three-dimensional objects, the process that is most economical in use of materials and that is also most advantageous in terms of design of machinery is the fused deposition modeling (FDM) process. This involves an extrusion-based, digital manufacturing system. There are also other known processes that are substantially analogous with slight differences, for example fused filament fabrication (FFM), melted extrusion manufacturing (MEM) or selective deposition modeling (SDM).
In the FDM method, two different polymer filaments are melted in a nozzle and are printed selectively. One of the materials involves a support material which is needed only at locations above which an overhanging part of the 3D object is printed and requires support during the subsequent printing procedure. The support material can be removed subsequently, e.g. via dissolution in acids, bases or water. The other material (the build material) forms the actual 3D object. Here again, the print is generally achieved layer-by-layer. The FDM process was first described in U.S. Pat. No. 5,121,329. Coloring is mentioned in general terms in US 2002/0111707, but is not described in any detail.
In the process described in EP 1 558 440, the individual layers are color-printed in a subsequent step. This process is slow, and printing of the thermoplastics that are already hardening leads to poorly resolved color definition.
In the 3D color-printing method in accordance with U.S. Pat. No. 6,165,406, separate nozzles are used for each individual ink. There are therefore only very restricted possibilities for ink mixing, and the color effect achieved becomes very simple.
In the FDM variant described in U.S. Pat. No. 7,648,664, variously colored build materials are used in granulate form, melted separately from one another, and mixed with one another in accordance with color requirement by an intervening extruder, before application as print. This method requires very complicated apparatus, and many advantages of FDM are lost.
In a very similar system according to EP 1 432 566, the mixing of the molten granulates is achieved directly in the heated printing head before these are directly applied as print. The said mixing can certainly not be complete, and the quality of print representation is correspondingly poor. Another disadvantage here is moreover that granulates or powders must be used and that these require separate storage and melting in the machine.
U.S. Pat. No. 6,129,872 describes a process in which the build material is melted in a nozzle and various colorant mixtures are metered selectively into the melt at the end of the nozzle. However, this leads to inadequate mixing and does not give clean color definition.
US 2010/0327479 describes a process in which a plurality of colored filaments are combined in a microextruder and are continuously extruded therein to give a new colored filament, which is then passed onward into the printing head for application as print. This process requires very sophisticated and complicated apparatus. The achievable color range is moreover subject to restriction resulting from the number of filaments. In an alternative embodiment, the variously colored filaments can also be conducted directly into the printing head, and mixed there. However, this variant also exhibits the disadvantages described above.
Therefore, an object of the present invention was to provide a 3D print process which can produce selectively colored and/or multicoloured three-dimensional objects with sharp and clear color definition.
Another object of the present invention was to provide an advantageous and rapid 3D print process for printing mechanically stable, multicolored objects.
Another object of the present invention was to provide a method having the ability to produce colored objects where the introduction of the color is not to be postponed to a downstream operation.
Other objects are not explicitly mentioned but are apparent from the entire context of the description, claims and examples below.